Auto parts cross the U.S.-Mexico border, sometimes more than once or twice, get added touches in one place and turned into modules at another. Subcomponents used in Illinois might rely on imported materials from Thailand or China or Mexico. Determining domestic content is an accounting morass.

The incoming administration's desire to bring more auto manufacturing jobs to the U.S. might lead it to a hard discovery: Trade negotiators have never been particularly good at building auto factories.

Beyond the ups and downs of poll numbers and delegate counts, there's a more interesting trend line to watch this year: South Carolina is changing, and it's doing so under the influence of the global auto industry.

Honda has decided to stop using airbags supplied by Takata Corp. That's a catastrophic turn of affairs for Takata after a run of quality glitches that have resulted in 19 million U.S. vehicle recalls and as many as eight deaths. But it's also a dramatic twist in what the auto industry has long assumed about Japanese keiretsus.